OSHA's Underground Construction Standard applies to the construction of underground tunnels, shafts, chambers, and passageways. It also applies to cut-and cover excavations connected to ongoing underground construction as well as those that create conditions characteristic of underground construction. These hazards include reduced natural ventilation and light, difficult and limited access and egress, exposure to air contaminants, fire, flooding, and explosion. The regulation does not apply to excavation and trenching operations for aboveground structures that are not physically connected to an underground construction operation or to underground electrical transmission and distribution lines.